Choose Logic Pro > Key Commands > Edit Assignments, click one of the commands to make sure the list of commands has the key focus, then press the spacebar a few times to see what it's assigned to. Is it assigned to Mute?
There's a quicker way which is:
Make sure you set the channel strip's input format to mono.
Command + Option + click the region to Marquee-select it.
Command + Shift + click a bit before the region start or a bit after the region end to extend the Marquee selection.
Press Command + J to join.
It's just two different Logic versions, but it's the same thing.
Basically, Soloing a track means muting all other tracks, but it does not mute tracks that are solo-safed, so that you can hear the solo-safed track(s) along with your soloed track(s).
Your 3rd party plug-ins are .component files found in one of these two locations:
Macintosh HD/Library/Audio/Plug-Ins/Components/
~/Library/Audio/Plug-Ins/Components/
That's a custom assignment.
The key command is Control + X in all Logic key command presets.
You can also access this function via menu: Functions > Remove Silence from Audio Region.
As a reminder, holding down Control while you choose an item in a menu opens the Key Command Assignments window with that command selected, so that you easily edit its assignment.